1. Field of the Invention
The present invention relates to electrical line protection circuits and more particularly to a line protection circuit for protecting a device in an ISDN network from damage due to power transients or power outages.
2. Description of Related Art
ISDN is an acronym for Integrated Services Digital Network, a type of public digital end-to-end telecommunications network which can support concurrent transmission of different types of information such as voice and data. The information is transmitted in digital form over two or more 64 kbps D channels. Use of the ISDN facilities is controlled through a common, low speed signalling channel known as a B channel.
In an ISDN network, device output signals are generated on a line including one winding of a transformer. The other winding of the transformer is connected to the remainder of the network. The transformer provides direct current isolation between the device and the remainder of the network.
Information is carried on the line by driving it at one of three distinct voltage levels, identified as 0+, 0- and 1. A "1" signal is the absence of any voltage across the ISDN line. A 0+ voltage is a predetermined voltage having a particular polarity while a 0- voltage is a voltage of the same magnitude but opposite polarity.
In known ISDN systems, a pair of diodes is connected to each of the two lines between each transformer winding terminal and its associated ISDN device to protect the device from power transients occurring elsewhere in the network. The anode of the first diode in each pair is connected to the line while its cathode is connected to a logic level voltage source, such as 5 volts. The cathode of the second diode is connected to the line while its anode is connected to ground. The diodes are reverse biased at normal ISDN signal levels. However, if a power transient causes the line voltage to increase significantly beyond the logic level voltage, the first diode becomes forward biased preventing the line voltage from significantly exceeding the logic level voltage. Correspondingly, if the line voltage begins to drop significantly below ground, the second diode becomes forward biased to prevent any further voltage drop.
Removing power from an ISDN device having diode protection circuits of the type described would result in ISDN signals being shorted to ground through the unbiased diodes. To prevent this, conventional practice is to include a normally open relay in the ISDN line between the diodes and the transformer winding terminal When the ISDN device is powered up, the relay is latched in a closed position using power supplied by the device. Powering down the device causes the relay to open, isolating the device and its unbiased diodes from the remainder of the network.
The use of a relay to provide device isolation presents a problem since a relay is an electromechanical device which is relatively costly in comparison to solid state devices. Moreover, the reliability of a relay or any other electromechanical device is always a concern because of the possibility that such devices may fail mechanically in an improper state.